TECHNOLOGY | WOOD PLASTIC COMPOSITES
defined a standard mixing process for most WPC applications using its system. He says that the machine automatically adjusts speed to control torque and temperature.
Testing for building Testing requirements for WPC in the US are governed by ASTM D7032, which was adopted into the US model building codes beginning in 2009, with only editorial updates since the original 2004 version, according to Craig Wagner, Chief Engineer for Intertek Building & Construction product certification and chair of the ASTM subcommittee on WPC decking. Wagner explains that ASTM D 7032 outlines
Above: Wood fibre filled PLA pellets and 3Dprint filaments produced in a Fraunhofer WKI development project
that wear and tear of the mixing system can affect consistency of the blend over time, so compound- ers should be aware of when maintenance is needed. Moisture of the natural fillers is also a considera-
tion, as these fillers are generally hydrophilic and easily absorb water if stored in a bag without a plastic liner. Before drying, they can contain more than 10% moisture in some cases, says Gehring. Mixaco’s HM/KM Wood mixing system is specifi- cally designed for WPC and provides a dehumidify- ing effect that reduces moisture. The system is comprised of an HM Heating
Mixer and KM High Efficiency Horizontal Cooler, adapted with other features for mixing of wood or natural fibres with PVC/PP/PLA or other polymers. “Even with a moisture above 10% [in incoming material] we can achieve a residual moisture of less than 1% in the final blend,” says Gehring. If moisture is not effectively removed, the
material will stick to surfaces and flowability of the blend is affected. “Moisture inside a WPC blend affects the process stability in further processing (for example, injection moulding, extrusion, or rotomoulding),” he says. Gehring adds that Mixaco has experimented with WPC mixing and has
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testing requirements for composite decking materials that include end-use factors (for example, flexural performance, load, creep recovery), biodeterioration, and surface burning characteris- tics. “IBC [International Building Code] Chapter 26 and IRC [International Residential Code] R507 both apply to deck boards, stair treads, handrails, and guards, and evaluate structural performance (span/ load rating), end-use factors (temperature and moisture effects), durability (weathering), and termite and decay resistance,” he explains. Wagner says that one area of regulation and testing that has expanded over recent years is the adoption of special fire resistance testing for applica- tion in areas prone to wildfires, commonly known as Wildland Urban Interface (WUI) requirements. Another aspect of testing that WPC processors should be aware of is the need to investigate the range of colours that might be used in a product. “This is not so much a concern for the colours (pigments) making a significant difference in the various test results, as it is more for making sure that you’re addressing differences in the base material compound. Sometimes one or more capstock colors are a different material from the others [for example, acrylic or PVC]. Another
IMAGE: FRAUNHOFER WKI/MANUELA LINGNAU
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